MAUNA LOA OBSERVATORY, Hawaii -- Two miles up, above black lava
fields and a white blanket of clouds, a tower rising from this U.S.
government observatory gulps in some of the clear, crisp air and gets
a taste of man's future on Earth.

"As big as the atmosphere is, we're influencing it," says the
physicist in charge, John Barnes.

The tale told by the tower, atop a dormant Hawaiian volcano, can be
read in the upward curve of a graph:

Carbon dioxide in the atmosphere, which stood at 280 parts per
million two centuries ago, has climbed to 379 parts per million since
industrializing man began burning vast amounts of coal, oil and other
fossil fuels.

There has not been, for 450,000 years, this much carbon dioxide
enveloping the planet, ice-core samples show.

The news from Mauna Loa and other monitoring stations has
increasingly disturbed scientists, because carbon dioxide traps heat,
as do other "greenhouse gases," and global temperatures have, indeed,
been rising -- by almost 1 degree Fahrenheit over a recent 18-year
period, a relatively rapid increase, NASA experts reported in April.

Warming will disrupt our climate, possibly drying out farmlands,
stirring up fiercer storms and raising ocean levels, among other
impacts, says the Intergovernmental Panel on Climate Change (IPCC), a
U.N.-organized network of hundreds of climatologists and other
researchers.

But the climate tale is far from simple. Earth's behavior -- physics,
chemistry, biology -- is an infinitely complex web of feedback loops,
reactions, recycling among the atmosphere, ocean, land and all their
components. Knowns are countered by unknowns, certainty by
uncertainty.

It was uncertainties that American oil, utility and other industries
pointed to in the 1990s in fighting international efforts to cap
fossil-fuel emissions. And President Bush cited the "incomplete state
of scientific knowledge" when he renounced the Kyoto Protocol, the
first step toward imposing those caps, in March 2001.

Then, just three months later, a National Academy of Sciences report
commissioned by the Bush White House supported the IPCC's finding.
Last year, two more prestigious organizations -- the American
Meteorological Society and the American Geophysical Union -- came to
similar conclusions.

Climatologists will never dispel the uncertainties "100 percent," but
they're working on it, and the Geophysical Union said computer
modeling of carbon, water and other cycles governing climate has
improved greatly in the past decade.

At universities and major centers worldwide -- such as the U.S.
government's National Center for Atmospheric Research in Boulder,
Colo., and Britain's Hadley Centre -- specialists peer into the
future via supercomputers, setting in motion vast global calculations
via thousands of interlocked mathemathical formulas.

Weather fronts flicker past on screens as temperatures and rainfall,
melting ice and ocean evaporation, cloud cover and a myriad of other
factors play out over days, months, years in "general circulation
models," or GCMs.

The leapfrogging of computer speed has boosted scientists'
confidence. But if computer power is meeting the challenge,
brainpower -- numbers of trained specialists, hands on keyboards to
input, minds to analyze -- is coming up short, scientists say.

"Climate change probably deserves a Manhattan Project-scale effort,"
said Scripps meteorologist Richard C.J. Somerville, referring to the
World War II atom-bomb project. "What there is, is a few dozen GCM
projects, each with a handful of people."

Whatever the resources, no one expects a "eureka moment" from the
modeling -- ironclad proof that last month's automobile exhausts
caused this month's warming. In fact, the added sophistication raises
new questions even as it helps answer old ones.

"All these little things now pop up. What about the size of
raindrops, what about sea ice, what about forests?" said senior
scientist Wallace Broecker of Columbia, who in the 1970s raised early
alarms about global warming. "We're going to have to make a decision
on what to do on the basis of insufficient evidence."

The uncertainty compounds the concern. For example, some believe
global warming will shrink "natural carbon sinks" -- that is, drought
will kill off rainforests, which absorb carbon dioxide. That would
raise levels of the gas in the atmosphere, worsening warming in a
dangerous circular feedback.

"If we get going now (on emission controls), we essentially buy time
for further research" on such questions, said Princeton University
climatologist Jorge L. Sarmiento.

Newer concerns focus on the unknowns of aerosols, or particulates --
tiny atmospheric particles of many kinds, from smokestack soot to
dust blown off the desert. Some particulates cool by scattering
sunlight, some warm. Some help clouds form, some break up clouds.

Atop Mauna Loa, amid the silvery domes of the National Oceanic and
Atmospheric Administration observatory, Barnes is researching
aerosols in the stratosphere, firing a laser's green beam into the
night sky to measure particles as far as 50 miles up.

Leading NASA scientist James Hansen believes climate models may have
missed a major particulate effect, from industrial soot accumulating
on snow and ice. Instead of reflecting almost all sunlight, the
darker landscape must be absorbing more heat, his team theorizes.

An aura of quiet urgency is building around climate research.

The tools deployed can dazzle, from distant space to ocean depths. A
NASA satellite, from 1 million miles away, will view the entire
sunlit Earth and measure its heat exchanges. To study ocean-climate
interaction, meanwhile, 20 nations are sending 3,000 technology-
packed floats as much as a mile deep in seas worldwide to drift with
currents for years, taking temperature and other readings, and
surfacing regularly to radio home the results via satellite.

Many believe the most urgently needed data will come from ICESat, a
newly launched satellite that for the first time will measure the
Greenland and Antarctic ice sheets, to monitor melting as the world
warms. Although full meltdowns would take centuries, runoffs could
raise the seas by dangerous inches in the coming decades.

Oceanographers report the North Atlantic is growing less salty
because of melt runoff and increased precipitation. This may be
ominous: It's the sinking of heavier, saltier water that draws warmer
surface waters from the south and keeps the climate warmer than
otherwise. A slowing of that ocean "conveyor" could drive down
northern temperatures.

This uncertain threat of a regional "ice age" was featured in an
internal Pentagon report in October, subtitled "Imagining the
Unthinkable." Describing a "plausible" though "not most likely"
scenario, it said abrupt climate change could stir conflict by
prompting mass migrations in search of shelter and food.

The extreme vision disturbed climatologists. "Irresponsible," one
said privately. But some hoped a Pentagon warning might arouse
concern in Washington, D.C.

"Maybe it's time for science fiction," said Harvard's Daniel Schrag,
an expert on Earth's ice ages. "The science is not exciting people."